JPH07195150A - Method for casting aluminum alloy for hdd - Google Patents

Abstract

PURPOSE:To provide a producing method of an aluminum alloy for HDD, by which the high strength aluminum alloy is obtd. and crystallized material having a specific value or higher can perfectly by eliminated. CONSTITUTION:The aluminum alloy composed of 5.0-8.0 Mg as the essential element and further, 0.03-0.3% Cu, 0.1-0.5% Mn and if necessary, further <=0.5% Zn and the balance with impurities, and regulated to 0.02-0.1% Fe and 0.02-0.1% Si in the impurities, is cast under the condition of 60-150mm/min casting speed and 200-500mm thickness of a cast block. By this cast method, the crystallized material having >=7mum particularly Al-Fe series crystallized material can perfectly be eliminated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a technique for manufacturing an aluminum alloy substrate for a disk, and more specifically, an HD mounted on a small computer such as a notebook or palmtop.
The present invention relates to a method for casting an aluminum alloy for D.

[0002]

2. Description of the Related Art Generally, as a base material for a magnetic disk used as a recording medium for a computer or the like, it is lightweight, non-magnetic and has rigidity, and good surface accuracy can be easily obtained by precision processing and polishing. Aluminum alloys have been used for reasons such as being available, being inexpensive, and the like. In particular, the 5000 series aluminum alloy such as the 5086 alloy has been used as an aluminum alloy base material for HDD through a coating type medium / thin film medium because of its excellent properties described above.

Usually, this aluminum alloy for discs is manufactured by the following steps. First, aluminum having a predetermined metal purity is melted, and various components such as Mg are added in appropriate amounts. After that, at a casting speed of 20 to 40 mm / min,
An ingot is made to a thickness of 200 to 600 mm to obtain an aluminum ingot. This ingot has 400
It is subjected to heat treatment at ˜550 ° C. This heat treatment is performed to make the ingot structure uniform. After that, hot rolling is performed 10
The plate should be less than mm. Further, it is cold-rolled and rolled to a thickness applicable as a disc material. After that, punching into a disk shape and further polishing the disk surface, so-called substrate processing, reduces surface roughness and removes oxides. Through the above steps, a so-called aluminum substrate is obtained.

This aluminum substrate is usually Ni-P plated on an Al substrate for surface hardening. However, when this plating treatment is performed, a plating defect such as a protrusion or a dent, a so-called nodule (a protrusion on the plating surface) or a pit (a dent on the plating surface) occurs on the plating surface. This plating defect is removed by plating film polishing in a subsequent step. However, it has been found that extremely fine plating pits are likely to occur at the sites where such defects occur. Such a pit has not been a cause of a data error in the past, but such a fine pit is becoming a cause of a data error with the recent increase in density. Therefore, in recent years, elimination of all plating defects has been studied.

As a result of diligent examination by the present applicant regarding this problem, there is a correlation between the number and size of plating defects and the number and size of crystallized substances on the basis of Al. As a result of pursuing the cause of plating defects, the first cause of plating defects in the crystallized substance is the diameter of 7 μm.
The above crystallized substance was investigated. As a result of further research, it was found that such a coarse crystallized substance becomes a plating defect for the following reason.

That is, in the case of a small crystallized substance having a size of 7 μm or less, it is dissolved and removed during the pretreatment of plating, and the Ni-
Since it is leveled in the P plating process, it does not cause a plating defect. However, when the crystallized substance is as large as 7 μm or more, the crystallized substance is not completely dissolved during the pretreatment of plating, and the crystallized substance remains in the Ni-P plating step. It has been clarified that is not a precipitate and becomes a defect.

This crystallized substance is M in the 5086 series.
It is known that there are g-Si and Al-Fe systems,
If the amount of Fe / Si decreases, the number of crystallized substances naturally decreases, and it becomes possible to reduce plating defects. So, for example,
Using high purity metal such as 99.99% purity, Fe ・ Si
It has been directed to reduce the crystallized size by reducing the amount. However, such a simple purification of the metal ingot not only increases the cost significantly, but also lowers the mechanical strength.

On the other hand, in recent years, the Al substrate is required to have high strength. This is because, in recent years, small computers such as notebook computers are forming a large market, and in order to support these computers, hard disks have recently been required to be thin and have a small diameter. For example, in terms of diameter reduction, 2.5 inch discs and other small diameter discs are forming a large market, and regarding thickness reduction, it is called thin-gage, an Al substrate that is about 30% thinner than conventional materials. Is beginning to be widely used.

At present, this thinning is further progressing. As a result of intensive studies by the present inventors in order to cope with this thinning, it was found that the conventional Al alloy cannot cope with the future thinning. This is because when the plate thickness becomes thin, the strength of the aluminum substrate drops sharply, so Ni-P
This is because the disk is deformed by the tension of the film. Further, for the same reason, there is a problem in that it is easily deformed during so-called handling, which makes manufacturing very difficult.

As described above, at present, there are two problems in aluminum discs: elimination of plating defects and enhancement of strength.

[0011]

The present applicant has conducted various studies in view of such circumstances, and as a result, increased the Mg content from the conventional 4% to 6% and the Mn to 0.3%. It was clarified that the addition of Al makes it possible to produce an Al alloy capable of achieving both high strength and small plating defect size.
(JP-A-56-105846). The feature of this manufacturing method is that hot rolling is performed at a high temperature in order to refine the crystallized substances. That is, the Mg-Si system crystallized substance is apt to cause precipitation and coarsening at a temperature of about 400 ° C. Therefore, it is utilized that the Mg-Si-based crystallized product generated during casting is soaked to form a solid solution and then hot-rolled at a high temperature to prevent coarsening of the crystallized product.

[0012] However, as a result of further study, it is possible to make the Mg-Si system crystallized product finer by the above-mentioned production method.
It was found that it was impossible to refine the l-Fe system crystallized product. This is because the Al-Fe-based crystallized substance has a very high melting point as compared with the Mg-Si-based crystallized substance, and the Al-Fe-based crystallized substance generated during casting is aluminum in the soaking / hot rolling process. This is because it cannot be dissolved as a solid solution. Further, the Al-Fe-based crystallized substance is generally coarser than the Mg-Si-based crystallized substance, and has a high probability of becoming a plating defect.

Therefore, it is necessary to control the Al-Fe system crystallized product so that the Al-Fe system crystallized product has a fine distribution in the casting process. Therefore, it has been conventionally proposed to apply a thin-wall casting method such as a continuous casting method of a thin plate for casting to a thickness of about 3 to 10 mm. This thin plate continuous casting method is described in detail in JP-A-60-140 and the like. However, such a thin plate continuous casting method has problems of extremely poor productivity and increased cost, and in recent years, it has become very difficult to apply it to a disc material. Further, when the thin plate continuous casting method is applied, since the rolling ratio is low, the crystal grains become coarse, so that there is a problem that the strength becomes lower than that of a normal material, and it is difficult to apply it as a thin material application.

Under the circumstances, the present invention achieves high strength and can completely eliminate crystallized substances of 7 μm or more.
It is an object to provide a method for producing an aluminum alloy for D.

[0015]

As a result of various studies by the present inventors in order to solve the above-mentioned problems, an increase in the casting speed is the most important for refining crystallized substances without increasing productivity and cost. The present invention has been made here by clarifying the desirability.

That is, in the present invention, M is used as an essential element.
g: 5.0-8.0%, Cu: 0.03-0.3
%, Mn: 0.1 to 0.5%, Zn: 0.5% or less if necessary, the balance consisting of Al and impurities, and Fe and Si of each of the impurities are 0. 0.02% ≤
An aluminum alloy regulated to Fe ≦ 0.1% and 0.02% ≦ Si ≦ 0.1% is cast at a casting speed of 60 to 150 mm / min and an ingot thickness of 200 to 500 mm. The gist is a casting method of an aluminum alloy for HDD, which has a high strength and a small crystallized product size.

[0017]

The present invention will be described in more detail below. First, the test results leading to the knowledge of the present invention will be described.

First, the present inventor investigated the soaking conditions, hot rolling conditions, etc., which affect the distribution of crystallized substances. As a result, as described above, it was found that the Mg-Si system crystallized product can be refined by improving the soaking and hot rolling steps, but the Al-Fe system crystallized product is affected only by the casting conditions.

Therefore, the casting conditions were examined. That is, a method called a DC casting method is generally applied to casting an aluminum alloy. In this method, cooling of the molten metal includes primary cooling with a water-cooled mold and secondary cooling with cooling water directly applied to an ingot. The cooling rate of the secondary cooling is higher than that of the primary cooling. However, with aluminum alloys, if water directly contacts the molten metal, an explosion will occur.
By the subsequent cooling, only the outside of the molten metal is cooled and solidified to form a so-called solidified shell. Then, water is directly applied to the solidified shell to carry out secondary cooling to prepare an ingot.

At this time, it was revealed that it is important to increase the cooling rate in order to make Al-Fe crystallized substances finer and to distribute a large number of them uniformly. That is, the crystallized material size depends on the growth rate of the crystallized material at the time of casting, and it is the diffusion amount of Fe atoms that controls the growth rate of the crystallized material. Therefore, in order to suppress coarse crystallized substances, it is necessary to complete the solidification before the Fe atoms in the molten metal are locally concentrated. When the casting speed is increased, the secondary cooling is performed within the high temperature of the molten metal, and the temperature gradient becomes steep. Therefore, the cooling rate and the progress rate of the solidification interface increase. Then, it was clarified that the molten metal can be solidified before Fe is locally concentrated and the crystallized substances can be made finer.

That is, according to the present invention, in order to increase the strength, M
The basic idea is to use an aluminum alloy in which a large amount of g and Mn are added and Fe / Si is regulated, and to eliminate plating defects, that is, to perform high-speed casting to refine crystallized substances. Next, the reasons for limiting the chemical composition of the Al alloy in the present invention will be explained.

Mg: Mg is the most effective element for improving the strength of the Al base, and is usually added in an amount of about 4% as a disc material. However, with this addition amount, sufficient strength cannot be obtained as a thin disk material, and 5.0% or more is required. However, if it exceeds 8.0%, even if the thin casting method is applied, problems such as segregation occur, and it cannot be industrially used.
Therefore, the Mg content is set to 5.0 to 8.0%.

Mn: Mn is an element necessary for promoting work hardening and raising the annealing recrystallization temperature. It also has the effect of reducing the size of plating defects. This is because a part of the coarse Al-Fe-based crystallized substance is replaced to form Al <Mn.Fe>, and as a result, the coarse Al-Fe-based crystallized substance is reduced. However, if the amount of Mn is less than 0.1%, this effect cannot be fully expected, and if it exceeds 0.5%, the etching property becomes excessive and the plated surface becomes very rough, which is not preferable. Therefore, the amount of Mn is 0.1
~ 0.5%.

Cu: Cu uniformly dissolves in the Al alloy, and has the effect of making the precipitation of Zn on the substrate surface during zincate treatment uniform and fine. Therefore, there is an effect of suppressing the generation of nodules on the plated surface, but if it is less than 0.03%, the above effect cannot be obtained, and if added in excess of 0.3%, the generation of nodules becomes large. Alternatively, the etching property of the crystal grain boundaries becomes excessive and the smoothness of the plated surface is impaired, which is not preferable. Therefore, the amount of Cu is 0.03 to 0.0.
3%.

Zn: Zn is also uniformly solid-dissolved in the Al alloy, and the precipitation of Zn on the substrate surface during zincate is made uniform and fine.
It has the effect of suppressing the generation of nodules. However,
It was found that the proper addition amount of Zn varies greatly depending on the plating conditions. That is, when Zn is not added, nodules can be reduced when plating is performed under conditions where plating pits are unlikely to occur, but conversely, when plating is performed under conditions where plating pits are likely to occur, Plating pits will occur. On the other hand, if the addition amount is too large, the etching property of the crystal grain boundaries becomes excessive, which causes plating defects. Therefore, Zn is not added, but if it is added, 0.5% or less is desirable.

Fe: Fe mixes in as a metal impurity and easily forms an Al--Fe intermetallic compound in a casting process or the like. This intermetallic compound falls off during processing as a disk substrate, such as cutting, polishing and grinding during so-called substrate processing, and becomes a dent, and it also falls off during the plating pretreatment process, causing pits on the plated surface. Becomes Therefore, the Fe content is preferably 0.1% or less. However, as described above, Fe is a constituent factor of the crystallized substance and is essential for increasing the strength. For that purpose, the minimum amount is 0.02%. Therefore, the Fe content is 0.02 to 0.1%.

Si: Si is also mixed as a metal impurity, and produces Mg-Si type crystallized substances in the casting process and the like. Since this Mg-Si system crystallized substance falls off in the pretreatment process of plating and causes pits, it is preferably 0.1% or less. However, as described above, Si is a constituent factor of the crystallized substance and is essential for increasing the strength. For that purpose, the minimum amount of 0.02% is necessary. Therefore, the Si content is 0.02 to 0.1%.

In the aluminum alloy for disks according to the present invention, Cr, Ti, other than these impurities,
B etc. may be contained in the range permitted by JIS5086 alloy.

Next, the casting conditions of the present invention will be described.

Casting speed: Most of the aluminum alloys used for disk applications are 5000 series Al-Mg alloys. Conventionally, this 5000 series alloy is usually ingoted at a casting speed of about 20 to 40 mm / min. This is because in the same alloy, the solid-liquid coexisting layer has a wide width, so that if the casting is performed at high speed, the solidified shell may be broken and the molten metal may leak.

However, as a result of diligent study by the present inventor, even if the casting speed is increased to 40 mm / min or more, if the amount of cooling water is sufficient and the casting conditions are appropriate, there is a possibility of molten metal leakage. It was confirmed that there was almost no. Further, as described above, the higher the casting speed, the finer the crystallized substances. This is because the faster the casting speed is, the higher the cooling rate is, and the faster the solidification interface progresses, so that the solidification of Al is completed before Fe is locally concentrated. Specifically, in order to set the maximum crystallized substance size to 7 μm or less, the casting speed is set to 6
It has been found that it is necessary to set it to 0 mm / min or more.
However, if the ingot is cast at a casting speed of more than 150 mm / min, the solidified shell may be broken and the molten metal may leak by the usual mold cooling method. Therefore, the casting speed is 60 ~ 150mm
/ Min.

Ingot thickness: The ingot thickness is 200 to 500 mm. If the thickness of the ingot is less than 200 mm, the rolling rate becomes low, the crystal grains become coarse, and the strength becomes low.
Further, when the thickness of the ingot exceeds 500 mm, the effect of improving the strength due to the rolling rate is saturated, and the cooling rate becomes low, so that the crystallized substance becomes coarse.

In casting under the above conditions, first, an aluminum alloy having an appropriate metal purity is melted at a melting point or higher, and Mg, Mn, Cu, etc. are added so as to have an appropriate content. Then, casting is performed thereafter to produce a so-called aluminum ingot. At this time, the casting speed is 60 mm / min or more and 150 mm / min or less as described above, and the ingot thickness is 200 mm or more and 500 mm or less. The manufacturing process after casting may be the same as the conventional one, and is performed, for example, under the following conditions.

The ingot is then subjected to soaking. This heat treatment is carried out in order to remove the casting strain and make rolling possible and to make the casting structure uniform. If the heat treatment temperature is 300 ° C. or lower, the homogenizing effect is not sufficient, and if the heat treatment temperature is 550 ° C. or higher, the surface of the ingot of Al easily burns and the surface quality is impaired.

After soaking, hot rolling is performed. It is important to carry out hot rolling at a temperature as high as possible (preferably 530 to 460 ° C.) to make the Mg—Si system crystallized product finer. Further, cold rolling is performed. In this step, the plate thickness is reduced to a thickness that can be used as a disc. Further, this rolled plate is punched, cut into a disk shape, and annealed as necessary to remove distortion.

Next, an ordinary rolled plate has a roughness of, for example, R.
Since a = 0.1 to 0.5 .mu.m is large as a disk substrate and it is necessary to further reduce distortion, the disk surface is removed by cutting or polishing to form a so-called substrate.

Base plating is applied to this substrate.
The plating process usually includes a pretreatment process such as degreasing, etching, and Zn substitution, and an actual plating film forming process. A nonmagnetic material such as Ni-P is usually used for the plating film. This plating step is performed to impart hardness to the disk surface and prevent damage due to head crush or the like when used as an HDD. Therefore, at least 5 μm for strength
It is desirable to set the plating thickness to the above, and if it is too thick, a large amount of plating solution is required, which is economically disadvantageous. By this plating step, a so-called base plating substrate can be manufactured.

This underplating substrate is then subjected to a polishing step to remove plating defects and obtain surface smoothness.
Further, so-called texture processing is performed as necessary, and a magnetic film is formed by sputtering or the like, and then used as a magnetic disk.

Next, examples of the present invention will be shown.

[0040]

Example 1 An aluminum alloy having the chemical composition shown in Table 1 was cast under the conditions shown in Table 2. At this time, the casting speed was increased while checking the surface state of the ingot during casting, and if there was a molten metal leak, the casting was immediately stopped and the speed was recorded. Then, after soaking at 510 ° C. for 18 hours, 48
Hot rolling was performed at 0 ° C. to obtain a thickness of 5 mm. Further, cold rolling was performed to prepare an aluminum plate having a thickness of 1.2 mm. This aluminum plate was punched into a disk having an outer diameter of 95 mm and an inner diameter of 25 mm, and the maximum crystallized product size was investigated. The results are shown in Table 3. From Table 3, it can be seen that under the casting conditions within the scope of the present invention, there is no crystallized substance of 7 μm or more.

[0041]

[Table 1]

[0042]

[Table 2]

[0043]

[Table 3]

[0044]

Example 2 An aluminum alloy having the chemical composition shown in Table 4 was cast at a casting speed of 80 mm / min and a cast thickness of 400 mm.
Then, after soaking at 510 ° C. for 10 hours, hot rolling was performed at 480 ° C. to obtain a thickness of 5 mm. Further cold rolling to 1.2 m
An aluminum plate with a thickness of m was prepared and its mechanical properties were investigated. Further, a disk having an outer diameter of 95 mm and an inner diameter of 25 mm was punched, then subjected to substrate processing, Ni-P plating was performed, and plating defects were investigated. Table 5 shows the results of the investigation of mechanical properties and plating properties.

The evaluation of the plating property was ◎ (excellent) when there were no plating pits and nodules of 10 μm or more, and 5 nodules of 10 μm or more without plating pits / mm ²
The following were evaluated as ◯ (good), and the other were evaluated as x (poor).

From Table 5, it can be seen that the inventive examples have much higher strength than the comparative examples and have no plating defects.

[0047]

[Table 4]

[0048]

[Table 5]

[0049]

As described in detail above, according to the present invention,
Since high strength can be achieved, crystallized substances of 7 μm or more can be eliminated, and high productivity is also possible.
The effect of contributing to the thinning of the aluminum base for HDDs and the reduction in diameter is remarkable.

Claims (3)

[Claims] 1. In% by weight (hereinafter the same), M as an essential element
g: 5.0-8.0%, Cu: 0.03-0.3
%, Mn: 0.1-0.5%, the balance consisting of Al and impurities, of which Fe and Si are each 0.0
An aluminum alloy regulated to 2% ≤ Fe ≤ 0.1% and 0.02% ≤ Si ≤ 0.1% is cast at a casting speed of 60 to 150 mm /
A casting method of an aluminum alloy for HDD having a high strength and a small crystallized product size, which is characterized in that the casting is performed under the condition that the ingot thickness is 200 to 500 mm. 2. The method according to claim 1, further containing Zn: 0.5% or less. 3. An aluminum alloy substrate for HDD, which is produced by the method according to claim 1.